1. Field of the Invention
The present invention relates to an original reading apparatus to be used in a facsimile system, an electronic file or a copying machine.
2. Description of the Prior Art
An image processing system has been proposed for electrically processing an image and displaying, printing or filing the image information. In order to input image information to be processed in such a system, an original image must be photoelectrically read and the image must be converted into electrical signals.
For this purpose, an original reading apparatus is generally used which employs an image sensor such as a CCD so as to read an image of an original placed on an original table. In an original reading apparatus, there is a demand for a multifunctional image reading apparatus having a function of printing image information always at the center of a recording medium for an image on any position of an original table, a function for suitably changing a magnification of a reproduced image in accordance with the sizes of the original and the recording medium, and so on. In order to provide such a multifunctional image processing apparatus, the position and size of an original placed on the original table must be detected. Such an apparatus is disclosed, for example, in commonly-assigned application (U.S. Ser. No. 539,461).
In this apparatus, a CCD is used as a line sensor as an image reading means to convert an original image into electrical signals, thereby reading image information. The position and size of an original placed on the original table are detected in accordance with a contrast difference between a portion of the original table on which the original is placed (original region) and a portion thereof on which no original is placed (non-original region). A signal corresponding to a black region is obtained corresponding to the non-original region, and a white region of the original is detected as the original region, so that the original and non-original regions can be discriminated. A counter for counting the output bit number in the main scanning direction from the line sensor and another counter for counting the line number in the subscanning direction are started in synchronism with the reference position of the original table. The counts in the main and subscanning directions obtained upon detection of a white signal of the original region are held or latched so as to allow detection of the position of the original placed on the original table.
FIG. 1 is a schematic representation illustrating the original reading method in the application (U.S. Ser. No. 539,461) referred to above. Referring to FIG. 1, an original M is placed on an original table S. The original M is read in the reading direction of the line sensor (main scanning direction ) X. The original table S is scanned in the subscanning direction Y. The original scanning is performed with reference to a reference position SP.
The position and size of the original M can be detected in accordance with a white signal corresponding to point P1 on the original M which is detected first, a white signal corresponding to point P2 which is closest to the reference point SP along the main scanning direction X, a white signal corresponding to point P3 farthest from the reference position SP along the main scanning direction X, and a white signal corresponding to point P4 of the original M which is detected last.
In this apparatus, an erroneous discrimination of the original region can be caused by a noise signal superposed on an image signal obtained by the reading operation of the line sensor, a white signal obtained due to the presence of a scratch on an original cover for keeping the original on the original table in position, contamination of the original table, or external light (to be referred to as noise components hereinafter). If such an erroneous discrimination of the original region can be prevented, the reliability of the apparatus can be improved further and the automatic recognition the original can be effectively performed.